. He full marks, but he was so careless as to make a spelling mistake.
A. must have gained
B. can have gained
C. could have gained
D. must gain
网友回答
A. must have gained
B. can have gained
(答案→)C. could have gained
D. must gain
解析：考查情态动词及虚拟语气。could have done本可能做某事而实际上没有做，句意：他本可以得满分的，但是他是如此的粗心以至于犯了一个拼写错误。所以选C。